1. Field of the Invention
The present invention relates generally to balancing gas turbine drive shafts, and more particularly to a balancing assembly which can be expanded inside a hollow drive shaft. The balance assembly can include an expandable balance weight carrier ring split for expansion against the inside surface of a hollow drive shaft, and is adapted to carry a plurality of balance weights.
2. Description of the Known Art
The main drive shafts of gas turbine engines are subject to mass imbalance due to manufacturing variations. The shafts must be balanced prior to assembly in the engine in order to reduce shaft vibration during engine operation.
Shaft imbalance is characterized by a magnitude of imbalance and an angular direction of imbalance. The magnitude of imbalance caused by an eccentric rotating mass (such as that formed by machining variations) is a function of the weight of the mass and the radial distance of the mass from the axis of rotation. The angular direction of imbalance is determined by the angular position of the eccentric mass relative to an arbitrary reference direction. The principle used in correcting the imbalance of a shaft is generally known and includes determining the magnitude and direction of shaft imbalance and generating an oppositely directed imbalance of the same magnitude. Means for determining the direction and magnitude of imbalance are well known by those skilled in the art.
Typically, shaft imbalance corrections are made at more than one axial location, or plane, along a shaft. Shafts are commonly balanced at three locations along the shaft, and it is desirable to be able to locate the balance correction assemblies at locations along the shaft where they will be most effective.
Accordingly, an ideal balance assembly would have a simple construction, provide a precisely variable correction to both imbalance magnitude and imbalance direction, and require minimum modification of a hollow shaft to allow positioning of the balance assembly along the hollow shaft where the balance assembly will be most effective.
One known method of balancing shafts that can provide variable imbalance correction requires grinding or machining material from raised lands on the outside surface of the shaft. However, since the maximum diameter of the shaft is limited by the opening in the engine core, the load carrying portion of the shaft must be reduced to allow for the raised lands. This results in an inefficient use of shaft diameter, increased shaft weight, and reduced stiffness of the load carrying portion of the shaft. In addition, machining the shaft may introduce areas of stress concentration that can reduce the shaft's load carrying capability.
A known method of balancing shafts that eliminates machining of shaft material is disclosed in U.S. Pat. No. 3,964,342 and divisional application 4,059,972, both issued to Beam et al. The patents disclose a balance tube assembly mounted within a hollow shaft. The balance tube includes exterior splined lands, each land holding a pair of balance weights with mating internal splines. Each balance weight mounted on the balance tube has a surface for locating the balance tube assembly inside a hollow shaft. Shaft imbalance is corrected by indexing paired balance weights relative to one another on each splined land prior to inserting the balance tube assembly inside a hollow shaft. The design requires a balance tube, which adds weight and complexity to the design. The location of the balance weights within the hollow shaft is limited by the location of the splined lands on the balance tube. The precision of the imbalance correction in the design is limited, since the imbalance correction is made by indexing the splined balance weights on the splined balance tube. An increased number of splines provides finer balance adjustment, but is also more expensive in terms of machining the balance tube and each balance weight.
Other internal shaft balance assemblies are known, such as those shown in U.S. Pat. No. 4,667,532 issued to Holz et al., and U.S. Pat. No. 4,455,887 issued to Lissajoux et al. However, these assemblies tend to be complex, and require internal machining of the shaft to accomodate the balance assembly. Other internal shaft balance assemblies shown in U.S. Pat. Nos. 4,043,147 and 4,539,864 issued to Wiebe require that a stiffening plate or plug be located in the shaft to accomodate the balance assembly. U.S. Pat. No. 4,539,864 also discusses the drawback of attaching a balance weight to one of the bolts which normally hold the stiffener plates in the shaft together, in that the precision of the balance direction angular adjustment is limited by the number of bolts in the stiffener plate.
As a result, gas turbine engineers continue to search for simple yet effective means for balancing gas turbine engine drive shafts.